A known form of electrical heating cable, e.g. from British Patent Specifications Nos. 2 098 438 A, 2 130 459 A and 2 138 660 A, comprises a pair of electrical conductors held separately or jointly in a core of electrical insulation material. A series of electrical heating wires are wrapped in a helical configuration or otherwise mechanically supported on the core and are connected in parallel across the conductors by contact with exposed portions of alternate conductors at spaced positions along the length of the cable. An outer insulating sheath is then applied over the core and heating wire. Since the heating wires must be wrapped around the core before the outer sheath is applied, this form of construction is very slow and expensive to manufacture without special machinery.
Although it is known to physically join the heating wire to the conductors e.g. by soldering, it is normally preferred for ease of manufacture to simply rely on pressure contact between the heating wire and the exposed conductor. Hence, in British Patent Specification No. 2 110 910 A it is proposed to use a heating element in the form of a heating wire wrapped around a bundle of glass fibre filaments, the whole heating element again being wrapped helically about an inner insulating core, to overcome the problem of loss of contact between the heating wire and the conductors when the wire expands. This form of construction is again relatively expensive to manufacture for the above-stated reason.
U.S. Pat. No. 4,117,312 again discloses a form of heating cable in which the heating wire is wrapped around an insulating core containing the conductors, and it also discloses a further form of construction in which a continuous strip heating element is deposited between a pair of conductors in continuous contact with them along the length of the cable. Continuation-in-part U.S. Pat. No. 4,072,848 discloses another arrangement in which discrete heating are arranged at spaced positions along the cable, each element being connected directly between the conductors on opposite sides of the element.
British Patent Specification No. 2 120 909 A discloses a form of cable which comprises two or more conductors, an inner insulating core, one or more heating wires and an outer sheath, but the conductors are not enclosed within the inner core. The conductors either run along opposite sides of the core and the heating wire is wrapped around the core and the conductors, or vice versa. In another illustrated embodiment the inner core is omitted altogether and a single heating wire zig-zags between the conductors These forms of cable are again difficult to manufacture without specialised machinery.
British Patent Specification No. 2 048 626 A discloses yet another form of heating cable in which a heating wire comprises a weft in a woven tape and the wire is periodically interwoven with two conductors running along opposite sides of the tape. An outer sheath is applied over the tape and the conductors but there is no inner insulating core surrounding the conductors. It is suggested that the woven tape could be replaced by a wire microhelix wound on a small diameter core but this postulated form of construction is not specified in further detail.
It is believed that the design of heating cables such as those described above has hitherto been constrained by the not unreasonable assumption that the heating element itself must be as close as possible to the external surface of the cable in order to provide a uniform heating effect from all surfaces of the cable and avoid possible local overheating of the heating element which could result in damage to the cable.
An object of the invention is to provide a form of heating cable which is quick and inexpensive to produce without requiring special machinery yet which is reliable and efficient in operation and has a good life expectancy.